1. Field of the Invention
The present invention relates to a pressure sensing module for use in catheters. More specifically, the present invention relates to a pressure sensing module that lends itself to easy installation and replacement, and provides a very low profile device for installation in a catheter.
2. Related Art
Catheter pressure transducers are long slender flexible tubular catheters with one or more pressure sensors mounted along the length of the catheter. These devices are used for making direct pressure measurements at various locations within a human or animal body by insertion through body orifices or through surgical incisions. The catheter pressure transducer may also be inserted through other catheters previously positioned within the body. These catheters include high fidelity pressure sensors for measuring pressure and pressure gradients. For example, catheter pressure transducers may be used in the various chambers of the heart and throughout the cardiovascular system. These catheters have also been used for measuring peristaltic activity in the intestines, pressure gradients in the urinary tract, rectal pressures, esophageal pressures, and intracranial pressures.
Present catheter pressure sensors typically take the form of photoelectric-type sensors, resistive-type sensors, or capacitive-type sensors. In the case of photoelectric-type transducers, light fibers pass the pressure signals from the sensor through the length of the catheter to a measurement device. Similarly, resistive-type and capacitive-type sensors use insulated electrical wires to pass pressure signals from the sensor through the catheter.
Exemplary sensors include small strain gauge-type pressure sensors that are made from a semiconductor material such as silicon. These semiconductor devices are generally shaped like a brick from which has been machined or etched a hollowed out region behind a diaphragm with diffused strain gauges for sensing the degree of diaphragm distortion under applied pressure. The sensors further include circuitry for providing electrical signals representative of the pressure sensed by the strain gauge to pads that may be connected to external devices. In operation, one side of the strain gauge diaphragm is equalized to a reference pressure, such as atmospheric pressure (the reference side), and the other side of the strain gauge diaphragm is exposed to the body tissues or fluids (sensing side). Such semiconductor strain gauge pressure sensors provide high-fidelity pressure measurements.
Traditionally, to mount the sensor within the catheter, a cavity is molded within the catheter or a premolded carrier is placed within the catheter to accept the sensor. When mounting the sensor, it is important to isolate the pressure sensor from mechanical movement and mechanical stresses that might sufficiently deform the sensor to give false pressure readings. The primary prior technique for mounting a semiconductor pressure sensor to a catheter has been to use a tubular metal casing, such as a portion of a stainless steel hypodermic needle. With this sturdy metal casing surrounding the sensor, the sensor is isolated from catheter movement and is sensitive only to external pressure. The proximal end of the metal casing is connected to the catheter body and is open to allow electrical connections from the catheter to reach the semiconductor pressure sensor. The proximal end is also open to allow venting of the back of the reference side of the strain gauge diaphragm to an external pressure through a lumen in the catheter body. With such prior devices, a window is generally cut into the side of the tubular metal casing. The sensing side of the strain gauge diaphragm of the sensor is left exposed through the window to applied external pressures from body fluids and tissues.
After mounting the sensor within the catheter, the wires from the sensor are bonded, either with a conductive epoxy or by soldering, to wires that have already been prepared within the catheter by an appropriate scraping or pretinning procedure. Wires within the catheter are typically 0.003″ in diameter, with wires from the sensor being typically approximately 0.001″ in diameter. These are all delicate wires to handle and to bond together. The sensor is usually attached in place with an epoxy resin or with a flexible RTV (room temperature vulcanizing silicone rubber).
When the mounting material has cured, a cleaning process is generally required to remove any flux or materials used in bonding the wires. Then, the wire connections are generally coated with an RTV or epoxy resin and the gap on the edges of the sensor between the sensor and the housing are generally sealed with an RTV. After these materials have been cured, an additional layer of epoxy and/or RTV is added to provide a smooth contour for the sensor area that will be compatible with the requirements for introduction into body vessels.
All these procedures are delicate and time-consuming and require skill, patience, and time to accomplish successfully. Because it is often difficult to apply an absolutely uniform coating of RTV, as required for consistency in pressure measurement, the results of the installation of the new sensor are not always what is required and the RTV may have to be modified or the sensor replaced again to meet stability requirements essential for good performance of a catheter pressure sensor. In addition, these installation procedures are typically performed directly within the catheter. This work is extremely delicate, time-consuming, and expensive.
In addition, prior catheter pressure transducers are either for single use, as sterile disposable items, or for multiple uses, wherein they will be cleaned and sterilized and reused for dozens and perhaps hundreds of times. Because the catheters are often very small in diameter (e.g., 0.5 mm to 3 mm), as for example when introduced into small vessels in the cardiovascular system, the pressure sensors are somewhat delicate devices and can be damaged during use.
Therefore a need exists for a device that lends itself to be easily installed during manufacture of the catheter and to be easily replaced as required. Further, a need exists for a pressure sensor that minimizes the problems and reduces the costs associated with traditional manufacturing procedures.